The present invention relates generally to polymer substrates, and more particularly to high temperature polymer substrates having improved properties.
There is a need for versatile visual display devices for electronic products of many different types. Although many current displays use glass substrates, there is a trend toward the use of plastic substrates. Plastic substrates are critical to future generations of electronic products and associated technologies because they are light weight, impact resistant, and cost effective. However, temperature limitations and gas and liquid permeation limitations of plastics have prevented their use in most displays.
Many processes in the manufacture of displays, such as flat panel displays, require relatively high temperatures that cannot be tolerated by most polymer substrates. For example, the recrystallization of amorphous Si to poly-Si in thin film transistors requires substrate temperatures of at least 160xc2x0-250xc2x0 C., even with pulsed excimer laser anneals. The conductivity of the transparent electrode, which is typically made of indium tin oxide, is greatly improved if deposition occurs above 220xc2x0 C. Polyimide curing generally requires temperatures of 250xc2x0 C. In addition, many of the photolithographic process steps for patterning electrodes are operated in excess of 120xc2x0 C. to enhance processing speeds in the fabrication. These processes are used extensively in the manufacture of display devices, and they have been optimized on glass and silicon substrates. The high temperatures needed for the processes can deform and damage a plastic substrate, and subsequently destroy the display. If displays are to be manufactured on flexible plastic materials, the plastic must be able to withstand the process conditions, including high temperatures over 100xc2x0 C., harsh chemicals, and mechanical damage.
Flexible plastic materials having a high glass transition temperature hold great promise for use in displays. As used herein, the term polymers having a high glass transition temperature is defined as those with a glass transition temperature greater than about 120xc2x0 C., preferably greater than about 150xc2x0 C., and most preferably greater than about 200xc2x0 C. Examples of such polymers include, but are not limited to, polynorbornene (Tg: 320xc2x0 C.), polyimide (Tg: 270-388xc2x0 C.), polyethersulphone (Tg: 184-230xc2x0 C.), polyetherimide, (Tg: 204-299xc2x0 C.), polyarylate (Tg: 148-245xc2x0 C.), polycarbonate (Tg: 150xc2x0 C.), and a high glass transition temperature cyclic olefin polymer (Tg: 171xc2x0 C., sold under the trade name Transphan(trademark), available from Lofo High Tech Film, GMBH of Weil am Rhein, Germany). Because of their temperature stability and high glass transition temperature, these materials offer promise in overcoming the temperature limitations of existing commodity polymers, such as polyethylene terephthalate (Tg: 78xc2x0 C.), and polyethylene naphthanate (Tg: 120xc2x0 C.).
However, polymers having high glass transition temperatures are often inherently mechanically weak, easily scratched, low in chemical resistance, and possess high oxygen and water permeability. Their poor properties make processing difficult. In addition, their high oxygen and water permeation rates, and poor surface finish preclude their use as substrates for sensitive display devices.
Many different display devices are presently being used, including liquid crystal displays (LCDs), light emitting diodes (LEDs), light emitting polymers (LEPs), electronic signage using electrophoretic inks, electroluminescent devices (EDs), and phosphorescent devices. Many of these display devices are environmentally sensitive. As used herein, the term environmentally sensitive display device means display devices which are subject to degradation caused by permeation of environmental gases or liquids, such as oxygen and water vapor in the atmosphere or chemicals used in the processing of the electronic product.
The gas and liquid permeation resistance of plastics is poor, often several orders of magnitude below what is required for sustained device performance. For example, the oxygen and water vapor permeation rates for polynorbornene and Transphan(trademark) are over 1000 cc/m2/day (at 23xc2x0 C.). The rate required to provide a sufficient lifetime for an organic light emitting device has been calculated to be approximately 10xe2x88x926 cc/m2/day (at 23xc2x0 C.). The environmental sensitivity of the display devices limits the lifetime, reliability, and performance of devices constructed on plastics, which has retarded the development of display devices made with plastic substrates.
Thus, there is a need for a high temperature substrate having improved properties, including ultra-low gas and liquid permeation, scratch resistance, and chemical resistance, which can be used as a support for display devices, and for methods for making such substrates.
The present invention meets these need by providing a high temperature substrate having improved properties and a method for making such a substrate. The substrate includes a polymer substrate having a glass transition temperature greater than about 120xc2x0 C., and at least one first barrier stack adjacent to the polymer substrate. The barrier stack includes at least one first barrier layer and at least one first polymer layer. The high temperature substrate optionally includes an environmentally sensitive display device adjacent to the first barrier stack and at least one second barrier stack adjacent to the environmentally sensitive display device. By adjacent, we mean next to but not necessarily directly next to. There can be additional layers intervening between the adjacent layers. The second barrier stack includes at least one second barrier layer and at least one second polymer layer.
Preferably, either one or both of the first and second barrier layers of the first and second barrier stacks is substantially transparent. At least one of the first barrier layers preferably comprises a material selected from metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof.
Either one of the first and second barrier layers can be substantially opaque, if desired. The opaque barrier layers are preferably selected from opaque metals, opaque polymers, opaque ceramics, and opaque cermets.
The polymer layers of the first and second barrier stacks are preferably acrylate-containing polymers. As used herein, the term acrylate-containing polymers includes acrylate-containing polymers, methacrylate-containing polymers, and combinations thereof. The polymer layers in the first and/or the second barrier stacks can be the same or different.
The high temperature substrate can include additional layers if desired, such as polymer smoothing layers, scratch resistant layers, antireflective coatings, or other functional layers.
The present invention also involves a method of making the high temperature substrate having improved properties. The method includes providing a polymer substrate having a glass transition temperature greater than about 120xc2x0 C., and placing at least one first barrier stack on the polymer substrate. The barrier stack includes at least one first barrier layer and at least one first polymer layer.
The barrier stack can be placed on the substrate by deposition or by lamination. The deposition is preferably vacuum deposition, and the lamination can be performed using an adhesive, solder, ultrasonic welding, pressure, or heat.
An environmentally sensitive display device can be placed on the first barrier stack, either by deposition or lamination. A second barrier stack can be placed on the environmentally sensitive display device. The second barrier stack includes at least one second barrier layer and at least one second polymer layer. The second barrier stack can be deposited on the environmentally sensitive display device, preferably by vacuum deposition.
Accordingly, it is an object of the present invention to provide a high temperature substrate having improved properties, and to provide a method of making such a substrate.